Liquid crystal display device and electronic device using the same

ABSTRACT

A liquid crystal display device includes an illuminator and a liquid crystal panel for performing displaying by using light which is emitted from the illuminator. The liquid crystal panel includes a pair of substrates, a liquid crystal layer provided between the pair of substrates, and a pair of alignment films provided on sides of the pair of substrates facing the liquid crystal layer. At least one of the alignment films is a photo-alignment film which is imparted with an orientation regulating force through a photo-alignment treatment, and the illuminator includes a light source causing primary generation of at least blue light, among other light which is used for displaying.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, and inparticular to a liquid crystal display device comprising an alignmentfilm which has been imparted with an orientation regulating forcethrough a photo-alignment treatment. Moreover, the present inventionalso relates to an electronic apparatus incorporating such a liquidcrystal display device.

2. Description of the Related Art

In recent years, liquid crystal display devices have been used in OAdevices such as personal computers and AV devices such as camcorders, onthe strength of being thin and consuming little power.

A liquid crystal display device performs displaying by utilizing opticalanisotropy of liquid crystal molecules, and therefore the orientationdirections of the liquid crystal molecules must be controlled withalignment films. As alignment films, films which are formed of a polymermaterial such as polyimide or polyvinyl alcohol and which have beensubjected to a rubbing treatment are commonly used.

However, when alignment films which have been subjected to a rubbingtreatment are used, there are problems in that orientation defects mayoccur due to foreign matter which emerged during the rubbing, orswitching elements (e.g., TFTs) which are provided on the substrate maybe destroyed by the static electricity which is generated during therubbing.

In order to solve these problems, a photo-alignment treatment(photo-alignment technique) has been proposed. A photo-alignmenttreatment is disclosed in Japanese Laid-Open Patent Publication No.2-277025 (Patent Document 1) or Japanese Laid-Open Patent PublicationNo. 4-303827 (Patent Document 2), for example. A photo-alignmenttreatment is a technique where an alignment film which is formed of acompound including a photoreactive functional group is irradiated withpolarized ultraviolet light in order to allow the molecules in thealignment film to undergo an anisotropic chemical reaction, whereby thealignment film acquires an orientation regulating force. Recently, therehave also been developed a method where non-polarized ultraviolet lightis used for the irradiation, instead of polarized ultraviolet light.

However, when a liquid crystal display device incorporating alignmentfilms which have been imparted with an orientation regulating forcethrough a photo-alignment treatment (hereinafter referred to as“photo-alignment films”) are used for long hours, the alignment may bedisturbed or the voltage retention rate may be lowered. Thus, a liquidcrystal display device incorporating photo-alignment films lacks inreliability.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andan objective thereof is to improve the reliability of a liquid crystaldisplay device incorporating a photo-alignment film.

A liquid crystal display device according to the present invention is aliquid crystal display device comprising an illuminator and a liquidcrystal panel for performing displaying by using light which is emittedfrom the illuminator, wherein, the liquid crystal panel includes a pairof substrates, a liquid crystal layer provided between the pair ofsubstrates, and a pair of alignment films provided on sides of the pairof substrates facing the liquid crystal layer; at least one of the pairof alignment films is a photo-alignment film which is imparted with anorientation regulating force through a photo-alignment treatment; andthe illuminator includes a light source causing primary generation of atleast blue light, among other light which is used for displaying. Thus,the aforementioned objective is met.

In a preferred embodiment, a spectrum of blue light which is emitted bythe light source has a peak wavelength at 380 nm or more.

In a preferred embodiment, the light source generates substantially nolight in an ultraviolet region.

In a preferred embodiment, the light source is a light-emitting diode.

In a preferred embodiment, the light source is an electroluminescenceelement.

In a preferred embodiment, the light source is a discharge tube.

In a preferred embodiment, the liquid crystal panel performs displayingin a vertical alignment mode.

In a preferred embodiment, the liquid crystal panel performs displayingin an in-plane switching mode.

In a preferred embodiment, the liquid crystal panel further includes aplurality of pixel regions each capable of modulating light emitted fromthe illuminator, and a switching element provided in each of theplurality of pixel regions.

In a preferred embodiment, the liquid crystal layer is formed of aliquid crystal material which contains molecules having at least one ofa carbon-carbon triple bond and a polycyclic group.

In a preferred embodiment, a coefficient of rotational viscosity γ₁ ofthe liquid crystal material at 20° C. is 120 mPa·s or less.

In a preferred embodiment, the molecules contained in the liquid crystalmaterial have a chemical structure expressed by one of the followingformulae:

(where n in the formulae is an integer equal to or greater than 2; andany hydrogen atom contained in a ring structure in the formulae may be,independently, substituted by a halogen atom, a cyano group, or anisocyano group).

In a preferred embodiment, the liquid crystal material contains 25weight % or more of the molecules having the chemical structure.

An electronic apparatus according to the present invention comprises aliquid crystal display device having the above construction. Thus, theaforementioned objective is met.

In a preferred embodiment, an electronic apparatus according to thepresent invention further comprises circuitry for receiving a televisionbroadcast.

An illuminator comprised in an liquid crystal display device accordingto the present invention includes a light source causing primarygeneration of at least blue light, among other light which is used fordisplaying, and therefore deterioration of the photo-alignment film dueto ultraviolet light is unlikely to occur. As a result, according to thepresent invention, the reliability of a liquid crystal display deviceincorporating a photo-alignment film can be improved, and thus a liquidcrystal display device which is capable of performing high-qualitydisplaying for long hours can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a liquid crystaldisplay device according to a preferred embodiment of the presentinvention.

FIG. 2 is a cross-sectional view schematically showing a liquid crystaldisplay device according to a preferred embodiment of the presentinvention.

FIG. 3 is a plan view schematically showing an active matrix substratewhich is used for a VA mode liquid crystal display device.

FIG. 4( a) is a diagram schematically showing how a photo-alignment filmis irradiated with ultraviolet light; and 4(b) is a diagramschematically showing a relationship between pretilt directions impartedto photo-alignment films and a tilting direction of a liquid crystalmolecule.

FIG. 5 is a plan view schematically showing an active matrix substratewhich is used for an IPS mode liquid crystal display device.

FIG. 6 is a plan view schematically showing an active matrix substratewhich is used for an IPS mode liquid crystal display device.

FIG. 7 is a graph showing an emission spectrum of blue LED #1 used for aprototype liquid crystal display device.

FIG. 8 is a graph showing an emission spectrum of blue LED #2 used for aprototype liquid crystal display device.

FIG. 9 is a graph showing an emission spectrum of blue LED #3 used for aprototype liquid crystal display device.

FIG. 10 is a graph showing emission spectrum of blue LED #4 used for aprototype liquid crystal display device.

FIGS. 11( a) and 11(b) are graphs showing an emission spectrum of acold-cathode tube (CCFL) used for a liquid crystal display device of acomparative example.

FIG. 12 is a graph showing a voltage-transmittance curve of a VA modeliquid crystal display device.

FIG. 13 is a graph showing a voltage-transmittance curve of a VA modeliquid crystal display device, where transmittance is shown in logarithmon the vertical axis.

FIG. 14 is a graph showing a voltage-transmittance curve of a TN modeliquid crystal display device.

FIG. 15 is a graph showing a voltage-transmittance curve of a TN modeliquid crystal display device, where transmittance is shown in logarithmon the vertical axis.

FIG. 16 is a graph showing an absorption spectrum of a TAC filmcontaining an ultraviolet absorber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The inventor has conducted a detailed analysis of the causes for theaforementioned problems occurring in a liquid crystal display devicecomprising photo-alignment films. Being a non-emission type displaydevice, a liquid crystal display device comprises an illuminator, anddisplaying is performed by modulating the light from the illuminatorwith a liquid crystal panel. The inventor has ascertained that a minuteamount of ultraviolet light is emitted from the illuminator, and foundthat the ultraviolet light deteriorates the photo-alignment films, thuscausing orientation disturbances and a decrease in the voltage retentionrate.

In the illuminator of a commonly-used liquid crystal display device, acold-cathode tube is used as a light source. In the cold-cathode tube,mercury which is enclosed within the tube is excited by discharging togenerate ultraviolet light, and this ultraviolet light excites aphosphor which is enclosed in the tube, whereby visible light that isused for displaying (which typically is light containing red, green, andblue light) is generated. In other words, the cold-cathode tube causesprimary generation of ultraviolet light, and the ultraviolet lightcauses secondary generation of visible light.

Not all of the ultraviolet light that is generated from the mercury isused for exciting the phosphor, but a part thereof is emitted outsidethe tube and reaches the liquid crystal panel. Although the ultravioletlight emitted outside the tube is so minute that it can hardly bedetected with a commonly-used illuminometer, the ultraviolet light willirradiate the liquid crystal panel for long periods of time, thuschanging the characteristics of the photo-alignment films and causingthe aforementioned problems.

In recent years, liquid crystal display devices have come to be used forliquid crystal television sets which display images from a televisionbroadcast. It is contemplated that a liquid crystal television set willbe placed in a living room or the like, and used for very long hours.Therefore, a liquid crystal television set is required to have areliability such that it is capable of performing stable displaying forabout forty thousand hours (10 hours/day×365 days×10 years). In suchlong hours of use, deterioration of the photo-alignment films due toultraviolet light from the illuminator presents a major problem.

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. Note that the present invention is notto be limited to the embodiment below.

FIG. 1 shows a liquid crystal display device 100 according to thepresent embodiment. The liquid crystal display device 100 comprises anilluminator 10A and a liquid crystal panel 20 which performs displayingby using light emitted from the illuminator 10A. A diffusion sheet 30for diffusing the light from the illuminator 10A is provided between theilluminator 10A and the liquid crystal panel 20.

The liquid crystal panel 20 includes: a pair of substrates (e.g., glasssubstrates) 20 a and 20 b; a liquid crystal layer 21 providedtherebetween; and a pair of alignment films 22 a and 22 b which areprovided on the sides of the pair of substrates 20 a and 20 b facing theliquid crystal layer 21. Although not shown in the figure, electrodesfor applying a voltage across the liquid crystal layer 21 are formed onthe substrates 20 a and 20 b.

Each of the pair of alignment films 22 a and 22 b is a photo-alignmentfilm which has been imparted with an orientation regulating forcethrough a photo-alignment treatment. As the photo-alignment films 22 aand 22 b, photo-alignment films which are formed by known techniques canbe broadly used. Although a case where both alignment films 22 a and 22b are photo-alignment films will be illustrated herein as a preferableembodiment, the effects of the present invention can be obtained so longas at least one of them is a photo-alignment film.

The illuminator 10A is an LED array which includes a plurality oflight-emitting diodes (LEDs) arranged in a matrix array as lightsources. Specifically, the illuminator 10A includes red LEDs 12R, greenLEDs 12G, and blue LEDs 12B.

Through recombination of electrons and holes occurring at a pn junctionwhere a bias voltage is applied in the forward direction, the red LEDs12R, green LEDs 12G, and blue LEDs 12B generate red light, green light,and blue light, respectively. In other words, the red LEDs 12R, greenLEDs 12G, and blue LEDs 12B cause primary generation of red light, greenlight, and blue light, respectively; and white light which containsthese kinds of light is radiated onto the liquid crystal panel 20 so asto be used for color displaying.

As described above, the illuminator 10A of the liquid crystal displaydevice 100 includes light sources causing primary (i.e., direct)generation of light to be used for displaying, and thereforedeterioration of the photo-alignment films due to ultraviolet light isunlikely to occur. Hence, orientation disturbances and decrease in thevoltage retention rate due to deterioration of the photo-alignment filmsare unlikely to occur, so that high-quality displaying can be performedfor long periods of time.

Although FIG. 1 illustrates an illuminator 10A which includes the redLEDs 12R, green LEDs 12G, and blue LEDs 12B, the present invention isnot to be limited thereto. An illuminator that includes blue LEDs andphosphors which absorb light from the blue LEDs and generate light inlonger wavelength regions may also be used. For example, an illuminatorthat includes blue LEDs and red LEDs as well as green phosphors whichgenerate green light by absorbing blue light, or an illuminator thatincludes blue LEDs, green phosphors, and red phosphors which generatered light by absorbing blue light may be used. By using an illuminatorthat includes light sources causing primary generation of at least bluelight, among other light which is used for displaying, deterioration ofthe photo-alignment films can be suppressed.

Moreover, although the illuminator 10A shown in FIG. 1 is a direct-typeilluminator in which LEDs are arranged in a matrix array immediatelyunder the liquid crystal panel 20, any other type of illuminator may beused. For example, as in an illuminator 10B shown in FIG. 2, it may be asidelight-type illuminator in which an LED 12 is disposed at an end faceof a light guide plate 14 that is provided at the rear face side of aliquid crystal panel 20 and in which light from the LED 12 is guided bythe light guide plate 14 into the liquid crystal panel 20.

The present invention can be suitably used for liquid crystal displaydevices of various display modes. For example, it may be suitably usedfor a liquid crystal display device of a twisted nematic (TN) mode, avertical alignment (VA) mode, or an in-plane switching (IPS) mode.

Now, a VA mode liquid crystal display device will be described. FIG. 3schematically shows an active matrix substrate 20 a of a VA mode liquidcrystal panel. Formed on the active matrix substrate 20 a are: aplurality of scanning lines 23 extending substantially in parallel toone another; a plurality of signal lines 24 extending in a directionintersecting the scanning lines 23; TFTs 25 electrically connected tocorresponding scanning lines 23 and signal lines 24; and pixelelectrodes 26 electrically connected to the TFTs 25. Each TFT 25 andeach pixel electrode 26 are provided in each one of a plurality of pixelregions arranged in a matrix array. On the active matrix substrate 20 a,storage capacitor lines 23′ for composing storage capacitors are furtherformed.

On the surface of the active matrix substrate 20 a shown in FIG. 3, aphoto-alignment film 22 a having vertical alignment properties isformed. As shown in FIG. 4( a), by irradiating the photo-alignment film22 a with polarized ultraviolet light, from a direction which is obliqueto the substrate-plane normal direction, its pretilt angle and pretiltdirection are controlled. Note that the “pretilt angle” is an anglebetween the major axis of a liquid crystal molecule whose orientation isregulated by the orientation regulating force of the alignment filmsurface and the substrate surface. The “pretilt direction”is anazimuthal direction of the major axis of a liquid crystal molecule whoseorientation is regulated by the orientation regulating force of thealignment film surface. Since the pretilt direction of a liquid crystalmolecule is defined by the orientation regulating force of an alignmentfilm, the direction of the orientation regulating force of an alignmentfilm is also expressed by the term “pretilt direction” in the presentspecification. As illustrated with respect to a lower left pixel in FIG.3, the photo-alignment film 22 a has different pretilt directions (solidarrows in the figure) respectively for four regions within the pixelregion.

Also, a photo-alignment film 22 b having vertical alignment propertiesis formed on the surface of a color filter substrate 20 b opposing theactive matrix substrate 20 a. By irradiating the photo-alignment film 22b with ultraviolet light, from a direction which is oblique to thesubstrate-plane normal direction, its pretilt angle and pretiltdirection are controlled. As shown in FIG. 3, the photo-alignment film22 b has different pretilt directions (dotted arrows in the figure)respectively for four regions within the pixel region. As shown in FIG.3 and FIG. 4( b), these pretilt directions are set so as to be oppositeto the pretilt directions of the photo-alignment film 22 a on the activematrix substrate 20 a side.

In the VA mode liquid crystal display device, liquid crystal molecules21 a contained in the liquid crystal layer 21 have a negative dielectricanisotropy such that, under an applied voltage, the liquid crystalmolecules 21 a having a negative dielectric anisotropy will be tiltedfrom a substantially vertical state. Since the pretilt directions of thephoto-alignment films 22 a and 22 b are set in the above-describedmanner, under an applied voltage, the liquid crystal layer 21 will formfour liquid crystal domains characterized by different orientationdirections of the liquid crystal molecules 21 a. In other words, eachpixel region is orientation-divided into four regions in which liquidcrystal molecules will tilt in different directions (four-dividedorientation). As a result of this, the viewing angle dependence ofdisplaying is reduced, whereby the viewing angle characteristics areimproved.

Next, an IPS mode liquid crystal display device will be described. FIG.5 schematically shows an active matrix substrate 20 a of an IPS modeliquid crystal panel. Formed on the active matrix substrate 20 a are: aplurality of scanning lines 23 extending substantially in parallel toone another; a plurality of signal lines 24 extending in a directionintersecting the scanning lines 23; TFTs 25 electrically connected tocorresponding scanning lines 23 and signal lines 24; and pixelelectrodes 26 electrically connected to the TFTs 25. The pixelelectrodes 26 are formed in the shape of combteeth extendingsubstantially in parallel to the signal lines 24.

On the active matrix substrate 20 a, common electrodes 27 are furtherprovided, which are formed in the shape of combteeth that aresubstantially parallel to the pixel electrodes 26. The common electrodes27 extend from common lines 28, which are formed substantially inparallel to the scanning lines 23. Via an insulative film (not shown),the common lines 28 oppose storage capacitor electrodes 29, which areformed of the same conductive layer as the pixel electrodes 26, and thusconstitute storage capacitors.

On the surface of the active matrix substrate 20 a shown in FIG. 5, aphoto-alignment film 22 a having horizontal alignment properties isformed. The photo-alignment film 22 a in the IPS mode is irradiated withpolarized ultraviolet light from the substrate-plane normal direction,so that hardly any pretilt will occur. The alignment direction of thephoto-alignment film 22 a is determined depending on the polarizationdirection of the ultraviolet light used for the irradiation.

Also, on the surface of the color filter substrate 20 b opposing theactive matrix substrate 20 a, a photo-alignment film 22 b havingvertical alignment properties is formed, and its orientation regulatingdirection is controlled by irradiating the photo-alignment film 22 bwith polarized ultraviolet light from the substrate-plane normaldirection.

In an IPS mode liquid crystal display device, liquid crystal moleculescontained in the liquid crystal layer 21 have a positive dielectricanisotropy such that, under an applied voltage, their orientationdirections are changed by lateral fields which are generated between thepixel electrodes 26 and the common electrodes 27 (electric fields whichare parallel to the layer plane of the liquid crystal layer). In an IPSmode liquid crystal display device, good viewing angle characteristicsare realized because the orientation directions of the liquid crystalmolecules vary within the plane which is parallel to the liquid crystallayer 21.

Note that the IPS mode has a problem in that a coloring phenomenonoccurs when observed in an oblique direction (a direction which istilted from the substrate-plane normal direction). Specifically, thelight becomes bluish when observed in the longitudinal direction of theliquid crystal molecules, whereas the light becomes yellowish whenobserved in the minor-axis direction of the liquid crystal molecules. Inother words, the light passing through the liquid crystal layer in anoblique manner (in a direction tilted from the layer normal direction)may become bluish or yellowish. This is because retardation of theliquid crystal molecules has a wavelength dispersion (wavelengthdependence).

In order to suppress the aforementioned coloring phenomenon, aconstruction as shown in FIG. 6 may be adopted. An active matrixsubstrate 20 a shown in FIG. 6 includes signal lines 24 which are bent aplurality of times (zigzag-shaped), as well as pixel electrodes 26 andcommon electrodes 27 which are bent so as to be substantially parallelto the signal lines 24 (in the “<” shape).

Since the pixel electrodes 26 and the common electrodes 27 have bentshapes as described above, under an applied voltage, two regionscharacterized by different orientation directions of the liquid crystalmolecules are created in each pixel region. Therefore, when observed ina certain oblique direction, each region causes the wavelength region oflight to be shifted to a hue of a complementary color, whereby thecoloring phenomenon is suppressed.

The inventor has actually produced liquid crystal display devices eachcomprising a liquid crystal panel having photo-alignment films and anilluminator including light sources causing primary generation of lightto be used for displaying, and evaluated their reliabilities.

First, the VA mode active matrix substrate 20 a shown in FIG. 3 and thecolor filter substrate 20 b were produced by known techniques. On thesurfaces of the active matrix substrate 20 a and the color filtersubstrate 20 b, an alignment film material whose main structure ispolyimide and which has a side chain that induces vertical alignmentproperties as well as a dimer photoreactive side chain was applied so asto form alignment films, and these alignment films were irradiated withpolarized ultraviolet light from a direction oblique to thesubstrate-plane normal direction. The active matrix substrate thusproduced was attached to the color filter substrate, and a liquidcrystal material was injected into the gap therebetween, thus producinga liquid crystal panel.

A plurality of liquid crystal panels as described above were produced,and on the rear faces of these liquid crystal panels, illuminators #1 to#4 having red LEDs, green LEDs and blue LEDs were provided, thusproducing liquid crystal display devices (Prototypes 1 to 4). Moreover,illuminator #5 having a cold-cathode tube (CCFL) was provided on therear face of a liquid crystal panel as described above, thus producing aliquid crystal display device (Comparative Example 1). The emissionspectra of blue LEDs #1 to #4 used for illuminators #1 to #4 are shownin FIG. 7 to FIG. 10, whereas the emission spectrum of the cold-cathodetube (CCFL) used for illuminator #5 is shown in FIGS. 11( a) and (b).Note that FIG. 11( b) is a graph obtained by magnifying the verticalaxis of FIG. 11( a) by 10 times. The peak wavelengths of blue LEDs #1 to#4 are shown in Table 1.

TABLE 1 LED #1 LED #2 LED #3 LED #4 peak 365 382 405 465 wavelength (nm)

The liquid crystal display devices of Prototypes 1 to 4 and the liquidcrystal display device of Comparative Example 1 were observed withrespect to aging. However, in order to conduct accelerated tests, theluminance of the light sources was set so as to be 10 times as large asthe usual luminance.

In the liquid crystal display devices of Prototypes 1 to 4, no changesoccurred after 500 hours. However, in the liquid crystal display deviceof Comparative Example 1, changes began to occur in the orientationdirections (pretilt directions) after 500 hours, and a decrease in thevoltage retention rate was also observed.

Moreover, in the liquid crystal display device of Comparative Example 1,changes in the orientation directions became greater after 1000 hours,and conspicuous display unevenness was observed. On the other hand,among the liquid crystal display devices of Prototypes 1 to 4, a slightdecrease in the voltage retention rate was observed for Prototype 1, butno change was observed for Prototypes 2 to 4.

The changes in the orientation directions and decrease in the voltageretention rate in the liquid crystal display device of ComparativeExample 1 are ascribable to the ultraviolet light which is generated bythe cold-cathode tube of illuminator #5. As shown in FIGS. 11( a) and(b), the emission spectrum of the cold-cathode tube exhibits peaks at313 nm (j line) and 365 nm (i line). These peaks are emission lines thatare characteristic of mercury emission, and are present in the emissionspectrum of a cold-cathode tube due to its principles. These emissionlines cause deterioration of the photo-alignment films, and thus lowerthe reliability. Since the main sensitive wavelengths of a commonly-usedphoto-alignment film are near about 250 to 320 nm, the light near the313 nm peak, in particular, greatly affects deterioration of thephoto-alignment film. On the other hand, blue LEDs #1 to #4 causeprimary generation of blue light, and thus the emission spectra of blueLEDs #1 to #4 do not have a peak at least near 313 nm, as shown in FIG.7 to FIG. 10. Therefore, the light which is generated by blue LEDs #1 to#4 is unlikely to allow the photoreactive functional group in thephoto-alignment film to react.

As described above, it has been confirmed that the reliability of aliquid crystal display device incorporating photo-alignment films isimproved by using an illuminator that includes light sources causingprimary generation of at least blue light, among other light which isused for displaying.

Note that, as can be seen from the fact that a slight decrease in thevoltage retention rate was observed in Prototype 1 after 1000 hours,from the standpoint of further improving the reliability, it ispreferable that the blue light which is generated by the light sourceshas a spectrum such that its peak wavelengths are at 380 nm or more(i.e., so as to fall within the visible region), as is the case withblue LEDs #2 to #4 of Prototypes 2 to 4. Moreover, it is more preferablethat the peak wavelengths are at 400 nm or more as is the case with blueLEDs #3 and #4, and it is further preferable that substantially no lightin the ultraviolet region is generated as is the case with blue LED #4.The reason is that, although the main sensitive wavelengths of aphoto-alignment film are contained within the aforementioned ranges, aslight sensitivity exists in other wavelength regions as well, and willbe integrated during hours of use of the liquid crystal television set(e.g., 40000 hours) to exhibit an influence. Moreover, in this case, thesensitive wavelength of a photo-alignment film which utilizes aphotoisomerization reaction of azobenzene may have an exceptionalpresence near 365 nm. When employing such a photo-alignment filmmaterial, blue LEDs #3 and #4 are particularly preferable, and it isfurther preferable that substantially no light in the ultraviolet regionis generated as is the case with blue LED #4.

Next, the active matrix substrate 20 a for the IPS mode shown in FIG. 5and the color filter substrate 20 b were produced by known techniques.On the surfaces of the active matrix substrate 20 a and the color filtersubstrate 20 b, an alignment film material having horizontal alignmentproperties (i.e., causing hardly any pretilt) was applied so as to formalignment films, and these alignment films were irradiated withpolarized ultraviolet light from the substrate-plane normal direction.The active matrix substrate thus produced was attached to the colorfilter substrate, and a liquid crystal material was injected into thegap therebetween, thus producing a liquid crystal panel.

A plurality of liquid crystal panels as described above were produced,and on the rear faces of these liquid crystal panels, illuminators #1 to#4 having red LEDs, green LEDs and blue LEDs were provided, thusproducing liquid crystal display devices (Prototypes 5 to 8). Moreover,illuminator #5 having a cold-cathode tube (CCFL) was provided on therear face of a liquid crystal panel as described above, thus producing aliquid crystal display device (Comparative Example 2).

The liquid crystal display devices of Prototypes 5 to 8 and the liquidcrystal display device of Comparative Example 2 were observed withrespect to aging. However, in order to conduct accelerated tests, theluminance of the light sources was set so as to be 10 times as large asthe usual luminance.

In the liquid crystal display device of Prototypes 5 to 8, no changesoccurred after 500 hours. However, in the liquid crystal display deviceof Comparative Example 2, changes began to occur in the orientationdirections (pretilt directions) after 500 hours, and a decrease in thevoltage retention rate was also observed.

Moreover, in the liquid crystal display device of Comparative Example 2,changes in the orientation directions became greater after 1000 hours,and conspicuous display unevenness was observed. On the other hand,among the liquid crystal display devices of Prototypes 5 to 8, a slightdecrease in the voltage retention rate was observed for Prototype 5, nochange was observed for Prototypes 6 to 8.

As described above, it has been confirmed that the reliability of an IPSmode liquid crystal display device incorporating photo-alignment filmsis improved by using an illuminator that includes light sources causingprimary generation of at least blue light, among other light which isused for displaying.

Note that the present invention is applicable to liquid crystal displaydevices of various display modes, and may be used for a TN mode liquidcrystal display device, for example, without being limited to the VAmode and IPS mode described above.

However, the effects of improving reliability were higher for the VAmode than for the TN mode. The reason thereof will be described withreference to FIG. 12 to FIG. 15. FIG. 12 and FIG. 13 are graphs showingvoltage-transmittance curves of a VA mode liquid crystal display device.FIG. 14 and FIG. 15 are graphs showing voltage-transmittance curves of aTN mode liquid crystal display device. The five curves shown in FIG. 12and FIG. 13 indicate, from the uppermost curve, cases where the pretiltangles are 87.9°, 88.4°, 88.9°, 89.4°, and 89.9°. The five curves shownin FIG. 14 and FIG. 15 indicate, from the uppermost curve, cases wherethe pretilt angles are 0.1°, 0.6°, 1.1°, 1.6°, and 2.1°.

As can bee seen from a comparison between FIGS. 12 and 13 and FIGS. 14and 15, and in particular from a comparison between FIG. 13 and FIG. 15where the transmittance is shown in logarithm on the vertical axis, thevoltage-transmittance curves at the black level to low-luminance grayscale levels (i.e., portions surrounded by broken lines in FIG. 13 andFIG. 15) are steeper and the amount of change in transmittance withrespect to changes in the pretilt angles is greater in the VA mode thanin the TN mode. Note that the gray scale level has an exponentialrelationship with transmittance. For example, the transmittance T_(n) atan n^(th) gray scale level in 256 gray scale-level displaying isexpressed as T_(n)=(n/255)^(2.2). Therefore, in order to discuss therelationship between gray scale levels and transmittance, it ispreferable to employ semi-logarithmic plotting as in FIG. 13 and FIG.15.

Since the amount of change in transmittance with respect to changes inthe pretilt angles is greater in the VA mode as mentioned above, in theVA mode, display unevenness may occur even if slight changes occur inthe pretilt angles due to deterioration of the photo-alignment films.Therefore, the reliability-improving effects of the present inventionare high. Moreover, in the case where orientation division is adopted,changes in the pretilt angles may cause changes in the positions of theboundaries between domains, whereby displaying coarseness may beobserved. Therefore, the reliability-improving effects are especiallyhigh in a VA mode where orientation division is adopted.

Moreover, the present invention provides high reliability-improvingeffects also in the IPS mode. In the IPS mode, displaying is performedby generating lateral fields using combteeth-like electrodes. However,since no lateral fields occur above the electrodes, the portions wherethe electrodes are formed do not contribute to displaying. Therefore,the effective aperture ratio is lower than that in the TN mode or the VAmode, and is typically about half of that in the TN mode or the VA mode.For this reason, in order to obtain the same luminance as in the TN modeor the VA mode, it is necessary to increase the brightness of the lightsources to about twice. If an illuminator including a cold-cathode tubeis employed as in the conventional case, deterioration of thephoto-alignment films is likely to occur. Hence, thereliability-improving effects of the present invention are high.

Furthermore, the reliability-improving effects of the present inventionwill also be clear in an FFS (fringe field switching) mode where, as inthe IPS mode, the orientation state of a horizontal alignment typeliquid crystal layer is controlled by using lateral fields.

The present invention is suitably used in a passive matrix-type liquidcrystal display device or an active matrix-type liquid crystal displaydevice, but provides clear effects especially in an active matrix-typeliquid crystal display device. In an active matrix-type liquid crystaldisplay device where a switching element (e.g., a TFT) is comprised ineach pixel, the charge which is charged in the pixel capacitance must beretained during one frame. If the photo-alignment films are deterioratedby ultraviolet light, the voltage retention rate will decrease, thusresulting in a lower display quality. According to the presentinvention, such a decrease in the voltage retention rate can besuppressed, and therefore active matrix driving can be performed in afavorable manner.

Note that ultraviolet light is also contained in external light enteringthe liquid crystal panel 20, and the light which is generated by blueLEDs may also contain a slight amount of light in the ultravioletregion. Therefore, in order to more certainly suppress the deteriorationof a photo-alignment film due to ultraviolet light, members forabsorbing ultraviolet light may be provided at the illuminator side orthe viewer side of the photo-alignment film, or members positioned atthe illuminator side or the viewer side of the photo-alignment film maybe formed from a material which absorbs ultraviolet light.

However, in a liquid crystal display device incorporating an illuminatorwhich includes a cold-cathode tube, deterioration of the photo-alignmentfilms will occur even if members for absorbing ultraviolet light areprovided. Polarizing plates having TAC (triacetyl cellulose) films,which contain an ultraviolet absorber, were used in the aforementionedPrototype- and Comparative-Example liquid crystal display devices, butdeterioration of the photo-alignment films nonetheless occurred in theComparative Examples. This is because even a member which absorbsultraviolet light cannot absorb all of the ultraviolet light that isgenerated upon light emission due to its principles.

FIG. 16 shows an absorption spectrum of a TAC film containing anultraviolet absorber. As can be seen from FIG. 16, this TAC film hasabsorptivity with respect to light of a wavelength of 400 nm or less.However, its OD (Optical Density) value is about 1 to 4, and thus it isnot able to completely absorb ultraviolet light. Therefore, evenultraviolet light which is so feeble that it cannot be detected by anilluminometer may, when radiated onto the photo-alignment film for longhours, reach a point where its cumulative energy unfavorably affects thephoto-alignment film.

Although the present embodiment illustrates LEDs as light sources, thisis not a limitation. Any light source can be broadly used which causeprimary generation of at least blue light. For example,electroluminescence (EL) elements can be used. Note that LEDs maysometimes be referred to as EL elements (EL element in the broad sense)because they perform light emission by utilizing electroluminescence.However, in the present specification, “EL elements” refer to intrinsicEL elements such as so-called organic EL elements and inorganic ELelements, and do not refer to injection-type EL elements such aslight-emitting diodes (LEDs), unless otherwise specified. An illuminatorthat includes red EL elements, green EL elements, and blue EL elementsmay be used, or an illuminator that includes blue EL elements andphosphors which absorb light from the blue EL elements and generatelight in longer wavelength regions may also be used. Alternatively, anilluminator that includes white EL elements in which red, green, andblue emission layers are overlaid may be used.

Moreover, it is even possible to employ discharge tubes which do notcause primary generation of ultraviolet light, such as neon tubesenclosing a noble gas causing primary generation of light which is usedfor displaying. For example, since neon is capable of primary generationof fire red and argon is capable of primary generation of blue-greenlight, a white light source can be obtained by combining a neon tube andan argon tube as well as a color filter for adjusting the color balance,for example.

The present invention is also suitably used in a liquid crystal displaydevice incorporating a liquid crystal layer which is formed of alow-viscosity liquid crystal material.

One technique for improving the response speed of a liquid crystaldisplay device is to employ a low-viscosity liquid crystal material.However, in a conventional liquid crystal display device, using alow-viscosity liquid crystal material may allow the voltage retentionrate to be lowered during use, thus resulting in display unevenness.

The inventor has analyzed the causes thereof, to find that a liquidcrystal material having a low viscosity is likely to contain moleculeswhich are susceptible to decomposition by ultraviolet light (i.e.,unstable against ultraviolet light), so that, when such molecules aredecomposed by the ultraviolet light from the light sources, a decreasein the voltage retention rate and display unevenness occur. Note that aliquid crystal material is generally a mixture of a plurality of typesof molecules (compounds), and any molecule composing the liquid crystalmaterial may not necessarily exhibit liquid crystal properties as asimple substance.

As in the present embodiment, by employing an illuminator that includeslight sources causing primary generation of at least blue light,decomposition of molecules due to ultraviolet light can be suppressed.As a result, decrease in the voltage retention rate and displayunevenness can be prevented. Hereinafter, low-viscosity liquid crystalmaterials containing molecules which are unstable against ultravioletlight will be specifically described.

When molecules having at least one of a carbon-carbon triple bond and apolycyclic group are mixed in a liquid crystal material, the viscosityof the liquid crystal material is lowered, whereby the responsecharacteristics of the liquid crystal display device can be improved.Although molecules having at least one of a carbon-carbon triple bondand a polycyclic group have low stability against ultraviolet light, thepresent invention suppresses decomposition of such molecules, wherebydecrease in the voltage retention rate and display unevenness can beprevented. In particular, when using a liquid crystal material whosecoefficient of rotational viscosity γ₁ at 20° C. is 120 mPa·s or less,there is a large significance in employing the present invention becausedecrease in the voltage retention rate and display unevenness are likelyto occur. Note that, in the present specification, “polycyclic groups”refer to both non-condensed polycyclic groups and condensed polycyclicgroups.

Examples of molecules having at least one of a carbon-carbon triple bondand a polycyclic group include molecules having a chemical structureexpressed by any of the following formulae. By mixing such moleculesinto the liquid crystal material, it can be easily ensured that thecoefficient of rotational viscosity γ₁ of the liquid crystal material at20° C. is 120 mPa·s or less. Note that n in the following formulae is aninteger equal to or greater than 2, and any hydrogen atom contained in aring structure in the following formulae may independently besubstituted by a halogen atom, a cyano group, or an isocyano group.

By mixing 25 weight % or more of molecules having any such chemicalstructure into the liquid crystal material, the viscosity of the liquidcrystal material is sufficiently lowered, whereby rapid response can beobtained. Specifically, a response time of about one frame or less canbe realized, and a level of moving picture performance that is requiredof a liquid crystal television set can be obtained.

Among molecules having the aforementioned chemical structures, moleculeshaving a tolan group (i.e., molecules including structures expressed bythe formulae shown by [formula 5] below, specific examples beingmolecules expressed by formulae (I) and (VI)) provide greatviscosity-reducing effects, and yet have very low stability againstultraviolet due to their triple bonds. Thus, the effects of the presentinvention will be most clearly exhibited for them.

Hereinafter, examples of liquid crystal materials and their constituentmolecules will be described more specifically.

As low-viscosity liquid crystal materials, liquid crystal materialscontaining molecules expressed by formula (I) below can be used, forexample. In formula (I), m and n are integers equal to or greaterthan 1. Liquid crystal materials containing molecules as expressed byformula (I) are disclosed in IDW '00, p. 77, for example, and can have acoefficient of rotational viscosity γ₁ of about 111 to 114 mPa·s at 20°C.

Alternatively, liquid crystal materials containing molecules which areexpressed by formula (II) below can be used. In formula (II), each of Aand B is independently a cyclohexylene, a phenylene, a phenylene some ofwhose H's are substituted by F's, or a cyclohexylene at least one ofwhose H's is substituted by D; at least one of Z₁ and Z₂ is —C≡C—; R1 isan alkyl, an alkenyl, an oxaalkyl, or an alkoxy (where preferably thenumber of C's is no less than 1 and no more than 10); and X₁, X₂, and X₃are H or F. Typically, X₂ is F, and at least one of X₁ and X₃ is F.

Liquid crystal materials containing molecules as expressed by formula(II) are disclosed in Japanese Laid-Open Patent Publication No.10-292173, for example, and can have a coefficient of rotationalviscosity γ₁ of 28 mPa·s or less at 20° C. Molecules expressed byformula (II) include structures expressed by the following formulae, forexample.

Alternatively, liquid crystal materials containing molecules expressedby formulae (III), (IV) and (V) below can be used. In formulae (III),(IV), and (V), R is an alkyl, an alkenyl, an oxaalkyl, or an alkoxy;each of X₁, X₂, X₃, and X₄ is, independently, H or F; and Y is F, —CF₃,—OCF₃, —OCHF₂, —OCH₂F, or R. Liquid crystal materials containingmolecules as expressed by formulae (III), (IV), and (V) are disclosed inJapanese Laid-Open Patent Publication No. 2002-38154, for example.

Moreover, liquid crystal materials containing molecules expressed byformula (VI) below can be used for an IPS mode liquid crystal displaydevice (having an active matrix substrate 20 a as shown in FIG. 5 orFIG. 6, for example). In formula (VI), m and n are integers equal to orgreater than 1.

Liquid crystal materials containing molecules as expressed by formula(VI) are disclosed in Japanese Laid-Open Patent Publication No.7-316556, for example. As is disclosed in this publication as Example 3,a liquid crystal material in which molecules expressed by formula (VI)and molecules expressed by formula (VII) are mixed has a coefficient ofrotational viscosity γ₁ of 20 mPa·s at 20° C.

Furthermore, liquid crystal materials containing molecules expressed byformula (VIII), (IX), and (X) below can be used for a VA mode liquidcrystal display device (having an active matrix substrate 20 a as shownin FIG. 3, for example). In formula (VIII), (IX), and (X), each of X₁ toX₆ is, independently, a hydrogen atom, a halogen atom, a cyano group, oran isocyano group. However, it is preferable that at least one of X₁, X₂and X₃, at least one of X₄ and X₅, and X₆ are not hydrogen atoms.Moreover, those of X₁ to X₆ which are not hydrogen atoms are preferablyhalogen atoms, and more preferably fluorine atoms.

Liquid crystal materials containing molecules as expressed by formula(VIII), (IX), and (X) are disclosed in Japanese Laid-Open PatentPublication No. 2002-69449, for example. A liquid crystal material whichis disclosed in this publication as Example 1 has a negative dielectricanisotropy, and can be used for a VA mode liquid crystal display device.

According to the present invention, the reliability of a liquid crystaldisplay device incorporating a photo-alignment film can be improved, anda liquid crystal display device which is capable of performinghigh-quality displaying for long hours is provided.

A liquid crystal display device according to the present invention canbe suitably used for various electronic apparatuses which are expectedto be used for long periods of time. For example, it can be suitablyused for a liquid crystal television set which includes circuitry forreceiving television broadcasts.

1. A liquid crystal display device comprising an illuminator and aliquid crystal panel arranged to perform displaying by using light whichis emitted from the illuminator, wherein, the liquid crystal panelincludes a pair of substrates, a liquid crystal layer provided betweenthe pair of substrates, and a pair of alignment films provided on sidesof the pair of substrates facing the liquid crystal layer; at least oneof the pair of alignment films is imparted with an orientationregulating force by an irradiation with ultraviolet light; theilluminator includes a light source causing primary generation of atleast blue light, among other light which is used for displaying; theliquid crystal layer is formed of a liquid crystal material whichcontains molecules having at least one of a carbon-carbon triple bondand a polycyclic group; and a coefficient of rotational viscosity γ₁ ofthe liquid crystal material at 20° C. is 120 mPa·s or less.
 2. Theliquid crystal display device of claim 1, wherein a spectrum of bluelight which is emitted by the light source has a peak wavelength at 380nm or more.
 3. The liquid crystal display device of claim 1, wherein thelight source generates substantially no light in an ultraviolet region.4. The liquid crystal display device of claim 1, wherein the lightsource is a light-emitting diode.
 5. The liquid crystal display deviceof claim 1, wherein the light source is an electroluminescence element.6. The liquid crystal display device of claim 1, wherein the lightsource is a discharge tube.
 7. The liquid crystal display device ofclaim 1, wherein the liquid crystal panel performs displaying in avertical alignment mode.
 8. The liquid crystal display device of claim1, wherein the liquid crystal panel performs displaying in an in-planeswitching mode.
 9. The liquid crystal display device of claim 1, whereinthe liquid crystal panel further includes a plurality of pixel regionseach capable of modulating light emitted from the illuminator, and aswitching element provided in each of the plurality of pixel regions.10. The liquid crystal display device of claim 1, wherein the moleculescontained in the liquid crystal material have a chemical structureexpressed by one of the following formulae:

(where n in the formulae is an integer equal to or greater than 2; andany hydrogen atom contained in a ring structure in the formulae may be,independently, substituted by a halogen atom, a cyano group, or anisocyano group).
 11. The liquid crystal display device of claim 10,wherein the liquid crystal material contains 25 weight % or more of themolecules having the chemical structure.
 12. The electronic apparatuscomprising the liquid crystal display device of claim
 1. 13. Theelectronic apparatus of claim 12, further comprising circuitry forreceiving a television broadcast.
 14. The liquid crystal display deviceof claim 1, wherein the liquid crystal panel is arranged to performdisplaying in a fringe field switching mode.
 15. The liquid crystaldisplay device of claim 1, wherein: the liquid crystal panel furtherincludes a plurality of pixel regions each capable of modulating lightemitted from the illuminator; and each of the plurality of pixel regionsis orientation-divided into four types of regions.
 16. The liquidcrystal display device of claim 1, wherein the illuminator is capable ofcausing primary or secondary generation of red light and primary orsecondary generation of green light.
 17. The liquid crystal displaydevice of claim 1, wherein the at least one of the pair of alignmentfilms is at least partially imparted with an orientation regulatingforce by an irradiation with ultraviolet light.